Photoluminescent materials and compositions that contain photoluminescent phosphorescent materials which have emissions in the visible region of the electromagnetic spectrum have been disclosed. For example, metal sulfide pigments which contain various elemental activators, co-activators and compensators have been prepared which absorb at 200-400 nm and have an emission spectrum of 450-520 nm. Further examples of sulfide photoluminescent phosphorescent materials that have been developed include CaS:Bi, which emits violet blue light; CaStS:Bi, which emits blue light; ZnS:Cu, which emits green light; and ZnCdS:Cu, which emits yellow or orange light.
The term “persistence” of phosphorescence is generally a measure of the time, after discontinuing irradiation, it takes for phosphorescence of a sample to decrease to the threshold of eye sensitivity. The term “long-persistent phosphor” historically has been used to refer to ZnS:Cu, CaS:Eu,Tm and similar materials which have a persistence time of only 20 to 40 minutes.
Recently, phosphorescent materials that have significantly higher persistence, up to 12-16 hours, have been reported. Such phosphors generally comprise a host matrix that can be alkaline earth aluminates (oxides), alkaline earth silicates, or alkaline earth aluminosilicates.
Although methods for uniquely marking and identifying objects have received thought and attention, such methods do not enable stealth detection. Thus, stealth detection refers to a method of identification wherein the emissions from the photoluminescent markings are not ordinarily observable by a human observer but detectable by friendly agents or friendly forces with specific detection equipment, and further wherein activation is not required during detection. This condition of not requiring activation during detection is important for the concept of stealth detection as the energy required for activation, such as by using a laser, is potentially detectable and hence revealing of the presence of the detector. Markings that enable stealth detection will be of high value in the combat theater for combat equipment or personnel. High persistence markings with emissions in the infrared region for stealth detection or stealth identification have been disclosed in U.S. Pat. Appl. No. 2008/185,557 and U.S. Pat. Appl. No. 2008/121,818 both to Agrawal et al.
Whether the photoluminescent markings enable stealth detection or not the specific marking itself may not be a stealth marking. Stealth markings are those wherein the markings themselves are created such that it is not readily apparent that the object has been marked for identification. This can only be achieved when the daylight color of the photoluminescent markings enable either the markings to blend in with the area surrounding the marking so as not to be distinguishable from the surrounding area, or wherein their daylight color is such that it is revealing of a marking that is normally present, such as a white or yellow color road marking. Photoluminescent markings that have emission in the infrared, such as those in referenced in U.S. Pat. Appl. No. 2008/185,557 and U.S. Pat. Appl. No. 2008/121,818, will generally be dark in color. Hence they can function as stealth markings only in very limited situations. The prior art is silent on how to make these markings function as stealth markings in a wide variety of environments.
In the field of markings for identification there is a need for markings that not only enable stealth detection or stealth identification but wherein the markings themselves are created such that it is not readily apparent that the object has been marked for identification, that is, the marking is a stealth marking. Such markings will be of great value in a variety of anti-terrorist activities, in the combat theater and also in anti-counterfeiting applications. The use of a functional overlayer, that is, a layer applied over the photoluminescent layer which contains carefully selected colorants can be of great value in adjusting the daylight color of photoluminescent markings that have emissions either partially or wholly in the infrared, whether or not the markings have been enabled for stealth detection, thereby camouflaging the presence of the marking.
Infrared emitting markings cited in U.S. Pat. Appl. No. 2008/185,557 and U.S. Pat. Appl. No. 2008/121,818 both to Agrawal et al have also disclosed the use an overlayer on top of the photoluminescent layer for the purpose of protecting the photoluminescent layer from physical damage and/or ensuring that any residual visible emission will not be detected. However there is no disclosure in these applications for the use of an overlayer to adjust the daylight color of the photoluminescent marking so as to render it as a stealth marking. These applications also do not disclose the use of an overlayer to enhance the daylight detectability of the photoluminescent markings. Nor do these applications disclose the use of an overlayer for the enhancement of photolytic stability of the underlying photoluminescent layer.
Photoluminescent materials, even if they are inorganic materials, can be subject to photolytic degradation. Even though ZnS phosphorescent materials have been known for a long time, their use for outdoor applications has been precluded because of photolytic instability. Additionally fluorescent materials, including fluorescent materials contemplated in this invention, can be sensitive to degradation by photolytic activity. Although use of UV absorbers, singlet oxygen scavengers, anti-oxidants, HALS (hindered amine light stabilizers) have been widely reported in the literature, such materials, by themselves, are not sufficient to impart the required photolytic stability of the markings of this invention to promote outdoor use of such markings.
It can therefore be seen from the above discussions that there is a need for photoluminescent markings whose emission is partly or fully in the infrared region of the electromagnetic spectrum, such emissions being suitable for stealth detection or not, wherein the markings have adjustable daylight color, or have improved photolytic stability, or have enhanced daylight detectability, or combinations thereof. There is also a need for methods of creating and using the inventive photoluminescent markings as well as objects containing the photoluminescent markings.